Mood and anxiety disorders such as major depressive disorder (MDD) and post traumatic stress disorder (PTSD) cause overlapping behavioral symptoms marked by hyperarousal, social avoidance, anxiety, increased startle responses and emotional numbing or diminished interest in pleasurable stimuli (anhedonia). A detailed understanding of the neural substrates and molecular mechanisms that mediate these symptoms will provide us with novel and more selective targets for drug development and ultimately increase the efficacy of treatment. Recent studies have provided strong evidence that extracellular signaling molecules, such as neurotrophic factors, glutamate and pro-inflammatory cytokines are elevated in patients with MDD and PTSD. Interestingly, all of these signals converge to activate the transcription factor nuclear factor ?B (NF?B), which has been suggested to play a role in the etiology of mood and anxiety disorders in humans. Using chronic social defeat stress, a mouse model of stress-related mood and anxiety disorders, we have observed an increase in NF?B activity in the nucleus accumbens (NAc), a key brain reward structure. Additionally, we found that chronic social defeat changes the morphology of NAc neurons that underlie the very long-lasting changes in behavior. Using a herpes simplex virus (HSV) expressing a constitutively active I Kappa Kinase (IKKca) to activate NF?B or a dominant negative I Kappa Kinase (IKKdn) to inhibit NF?B, we show that expression of IKKca in the NAc of nave mice mimics the anxiety phenotype produced by chronic social defeat. In addition, and consistent with findings in defeated mice, IKKca increases dendritic spine number and IKKdn decreases dendritic spine number on NAc medium spiny neurons. Although the biochemical mechanisms of NF?B mediated spine alterations are unknown, intracranial injections of the HSV-IKK mutants into the NAc resulted in gross changes in Rac1-PAK1 signaling, a RhoGTPase pathway known to mediate actin cytoskeletal reorganization and the development of new spines. Inhibition of NF?B with IKKdn decreases activity within the Rac1-PAK1 pathway, whereas, IKKca greatly increases its activity. Interestingly, social defeat also reduces activity of Rac1 and PAK1 in the NAc, and inhibition of Rac1 signaling with a dominant negative mutant, increases susceptibility to stress, further highlighting the importance of these biochemical changes in producing social defeat-induced avoidance. Based on the results thus far, we believe that chronic physical and psychological stress-induced changes in spine density underlie certain aspects of the social defeat behavioral syndrome. We are also further examining whether these stress-induced increases in dendritic spines and behavior are via NF?B regulation of RhoGTPase signaling.